The most widely used form of retroreflective sheeting is the "enclosed-lens" or "embedded-lens" form originally taught in U.S. Pat. No. 2,407,680. Such sheeting includes a transparent polymeric sheet, a monolayer of microspheres embedded within the sheet, and a specularly reflective layer underlying the back surface of the polymeric sheet. To achieve maximum reflection from such a sheeting, the distance between the microspheres and the specularly reflective layer must be closely controlled to place the latter at the approximate location where light rays are focused by the microspheres. The conventional approach to providing such a controlled spacing is to coat the microspheres with a polymeric layer, known as the spacing layer, prior to application of the specularly reflective layer.
A problem with this conventional coating approach is that the coated layer is generally at an optimum thickness only in a small area directly in back of individual microspheres. The coated material tends to flow into the areas between the microspheres, giving those areas an enlarged thickness and spacing the specularly reflective layer away from the focal point of the microspheres. Incident light that is perpendicular to the sheeting, or only slightly displaced from perpendicular, is brightly reflected since it is focused onto the small areas of optimum spacing in back of individual microspheres. But light impinging on the sheeting at an angle substantially displaced from perpendicular is focused to a point in front of the specularly reflective layer, and retroreflection at those angles is reduced.
Despite the recognized disadvantage of this limited "angularity" of conventional embedded-lens retroreflective sheeting, no significant improvement in the angularity of commercial embedded-lens sheeting has been obtained for many years. The sheeting continues to have half-brightness angles (the angle at which light incident on the sheeting is reflected at half the brightness that light perpendicular to the sheeting would be reflected) of about 30.degree. to 45.degree.. Such an angularity is adequate for many purposes, but not for other potentially important uses such as signing on the sides of trucks or other vehicles. Motorists often view a sign on the side of a truck from a position other than perpendicular to the truck, at large incidence angles far beyond the angles at which existing embedded-lens reflective sheetings are reflective.
U.S. Pat. No. 4,367,920 teaches retroreflective sheeting products of improved angularity, and processes for making such sheeting, involving embedding a monolayer of microspheres into a polymeric layer to less than one-half the average diameter of the microspheres, and laminating a preformed spacing film to the microsphere-covered surface of the first layer. The present invention adds to that teaching to further advance retroreflective sheeting to a state of greatly enhanced angularity.
In brief summary, the basic method of manufacture comprises the steps of preforming, preferably by extrusion, a first transparent polymeric layer; embedding a monolayer of microspheres in the layer under heat and pressure to a depth less than one-half the average diameter of the microspheres; preforming, preferably by extrusion, a second transparent polymeric layer and laminating the second layer to the microsphere-covered surface of the first layer so that the second layer follows the curved surfaces of the portions of the microspheres protruding from the first layer and contacts the first layer in the spaces between the microspheres; and coating the exposed configured surface of the second layer with a specularly reflective layer. Preferably, according to the present invention, the microspheres are embedded into the first layer to depths that leave the extreme edges of the non-embedded portions of the microspheres in substantial alignment; the microspheres are applied in lower numbers per unit area than might otherwise be obtained; and the microspheres are used in a broader range of diameters than generally regarded as optimal in the past, all to obtain further improvements in angularity. Also, the lamination is preferably achieved by use of a cushioning web comprising a polymeric material which engages the second polymeric layer and which softens during the lamination step to a softer or lower viscosity condition than the second polymeric layer.
It has been found that sheeting prepared in the manner described has an angularity never before achieved in an embedded-lens retroreflective sheeting. For example, the half-brightness angle for sheeting of the invention is generally 50.degree. or more, and preferably 60.degree. or more, on at least one axis of the sheeting, in contrast to the conventional half-brightness angle of about 30.degree. to 45.degree. noted above. Also, the new sheeting is reflective to very high incidence angles approaching 90.degree., whereas conventional embedded-lens sheetings have little if any reflection at angles of incidence greater than about 65.degree..
While not restricting ourselves to a particular mechanism or theory, it is believed that the superior angularity of the new sheeting can be attributed at least in part to the fact that in such sheeting the spacing layer conforms in a substantially constant thickness around a large portion of the back surface of the microspheres. Because of the shallow embedding of the microspheres in the first layer, there is a large unfilled space between the microspheres, which can accommodate excess portions of the spacing layer during lamination of the spacing layer to the microspheres, and thus avoid an accumulation of the material of the spacing layer that would otherwise thicken the spacing layer over portions of the back surface of the microspheres. Also, alignment of the back surfaces of the microspheres allows the spacing layer to be applied more uniformly to individual microspheres irrespective of the sizes of the microspheres. Control over the density per unit area of the microspheres further enhances conformation of the spacing layer, as does use of a softenable cushioning web during the lamination operation.